A motor driving device that drives a motor as an inductive load by operating a semiconductor switching element such as a MOSFET connected in series with the motor under a PWM control has been known. In such a motor driving device, further, it has been proposed to have a function of detecting the disconnection of the motor. For example, the disconnection of the motor is detected based on a fact that an electric current for the motor does not flow and a PWM signal is not outputted toward the motor when the motor is disconnected. In such a detection, however, in a case where the motor is employed as a fan motor of a vehicle, if the motor rotates due to disturbance such as the wind, an electromotive force is generated at a winding of the motor. As a result, the disconnection is erroneously detected because PWM oscillation to the load is seemingly stopped.
For example, JP2001-298988A, which corresponds to U.S. Pat. No. 6,512,346, describes a motor driving apparatus having a disconnection detection function. In the described apparatus, a resistor element is connected in parallel with a transistor that drives a motor, and a voltage at a common connecting point between the motor and the transistor is monitored. When a state where the voltage does not fluctuate according to a control signal (PWM signal) continues over a predetermined period of time, it is determined that the motor is disconnected. Further, when a driving voltage detected by an average voltage detection circuit exceeds a target driving voltage, the output of the control signal is stopped, whereby to avoid erroneous detection.